Thermal Dissipation Probes Research Articles

Sap flow and responses to meteorological about the Larix principis-rupprechtii plantation in Gansu Xinlong mountain, northwestern China

The transpiration of trees plays a decisive role in the water balance of forest stands and in water yields from forested catchments, especially an artificial forest planted for the purpose of soil and water conservation in semiarid mountain ecosystem regions. For proper management of watershed ecosystems, quantification of water use, sensitivity and adaptation for artificial forest is needed. This study were monitored the sap flow for Larix principis-rupprechtii plantation by thermal dissipation probes (TDP), analyzed the correlation between the sap flow and the meteorological factors on daily and seasonal scale, and upscaled stand transpiration from individual sap flow measurements. Within a daily timescale, the net radiation (Ra), air temperature (AT), air humidity (AH), vapor pressure deficit (VPD) and wind speed (WS) showed a significantly positive correlation with sap flow velocity in day-time, but in the night-time sap flow had no significant correlation. Within a monthly timescale, the Ra, AT, WS, and VPD showed a significantly positive correlation with stand water use, whereas precipitation (Pr) showed a significantly negative correlation with stand water use. Our results also showed that the daily transpiration of L. principis-rupprechtii continued to increase with the increase of net radiation (Ra), until the late stages of growth season (except rainy and cloudy days), and the sap flow velocity showed an obvious hysteresis response to meteorological factors to alleviate plant water stress from June to October, while there was no significant correlation between day-time and night-time water use during the stage of germination and rapid growth in May. Besides, due to the relatively low temperature, the water consumption of forest stands was only about 151.05 mm during the whole growing season, and there was no obvious ‘noon break’ phenomenon in the whole growing season. These results indicated that L. principis-rupprechtii could manage the water consumption conservatively, and were appropriate for afforestation in the mountains ecosystem with high altitude and low temperature.

Time lag of stem sap flow and its relationships with transpiration characteristics in Quercus liaotungensis and Robina pseudoacacia in the loess hilly region, China

Using Granier-type thermal dissipation probes (TDP), we measured stem xylem sap flow of the natural dominant species Quercus liaotungensis and a reforestation species Robinia pseudoacacia from July to September in 2016 in the semiarid loess hilly region. Meteorological factors and soil water content were simultaneously monitored during the study period. Using cross-correlation analysis, time lag between diurnal patterns of sap flux density and vapor pressure deficit (VPD) was quantitatively estimated. Differences in the time lag between the two species and possible influence by different diameter classes and soil water contents were analyzed. The results showed that the diurnal courses of sap flux density were similar to those of meteorological factors, with daily peaks ear-lier than VPD. The peak of VPD lagged behind the sap flux densities of Q. liaotungensis and R. pseudoacacia 118.2 min and 39.5 min, respectively. The peak of PAR lagged behind the sap flux density of Q. liaotungensis 12.4 min, but was 68.5 min ahead of that for R. pseudoacacia. Time lag between sap flux density and VPD significantly varied between tree species and was affected by soil water content. Those during higher soil water content period were about 32.2 min and 68.2 min longer than those during the period with lower soil water content for the two species, respectively. There was no correlation between time lag and tree diameter classes. The time lag between VPD and sap flux density for R. pseudoacacia was about 21.4 min longer in smaller diameter trees than in larger trees, which was significantly different under the lower soil water content. Our results suggested that the time lag effect between VPD and sap flux densities in the two species reflected their sensitivities to driving factors of transpiration, and that higher soil water content was favorable to sap flux density reaching its peak early. The lower soil water content might lead to lower sensitivity of the trees to meteorological factors. R. pseudoacacia was more sensitive to changes of soil water content.

Diurnal and nocturnal transpiration behaviors and their responses to groundwater-table fluctuations and meteorological factors of Populus tomentosa in the North China Plain

For theories of forest water utilization over day-night cycles, it is important to have a reliable information on plant transpiration behaviors during daytime and nighttime and their environmental controls. With the help of thermal dissipation probes, our study investigated the seasonal variations in the diurnal (Qday) and nocturnal stand water use (Qnight) of a Populus tomentosa plantation and explored their responses to groundwater table (GT) fluctuations and the meteorological factors. The results indicated that Qnight occurred on most nights during the growing season in the P. tomentosa plantation, but with a different seasonal dynamic from Qday. A distinct increase in the ratio of nocturnal to daily water use appeared at the beginning and the end of the growing season. During the experimental period, stem refilling accounted for 61% of Qnight, indicating that P. tomentosa was able to draw on stored water in the stem to support transpiration. The Qday and Qnight were positively and negatively related to GT, respectively. When the GT varied within 170 cm of the surface, the proportion of stem refilling accounting for Qnight increased with GT. The positive and negative correlations were found in meteorological factors with Qday and Qnight, respectively. Among the single meteorological factors except for ET0, vapor pressure deficit (VPD) and photosynthetically active radiation (PAR) explained the highest variation in Qday in 2010 and 2011, respectively. In contrast, Qnight was dominantly affected by VPD in 2011. Nocturnal stand water-use could help P. tomentosa to recharge the consumption of water stored in the stem and overcome the seasonal drought. The compensation effect of groundwater on the nocturnal stand water-use of P. tomentosa could reduce its consumption of water stored in the stem. As the stand age of P. tomentosa plantation varied, the key meteorological drivers of both Qday and Qnight also changed. Our results enable better understanding of the water-use strategies of poplars in the North China Plain and can lead to refinement of current silvicultural techniques.

Water Transfer Between Bamboo Culms in the Period of Sprouting.

Bamboo culms are connected to neighboring culms via rhizomes, which enable resource exchange between culms. We assessed water transfer between established and neighboring, freshly sprouted culms by thermal dissipation probes (TDP) inserted into culms and the connecting rhizome. During the early phase of sprouting, highest sap flux densities in freshly sprouted culms were observed at night, whereas neighboring established culms had high sap flux densities during daytime. After leaf flushing on freshly sprouted culms, the nighttime peaks disappeared and culms switched to the diurnal sap flux patterns with daytime maxima as observed in established culms. TDP in rhizomes indicated water flowing from the established to the freshly sprouted culms. When the established culms of a clump were cut, freshly sprouted culms without leaves reduced sap flux densities rates by 79%. Our findings thus suggest that bamboos exchange water via rhizomes and that nighttime fluxes are highly important for the support of freshly sprouted culms. The (water) resource support may facilitate the very fast growth of the bamboo shoots, and enable the colonizing of new places.

Calibration of Granier-Type (TDP) Sap Flow Probes by a High Precision Electronic Potometer.

Thermal dissipation probe (TDP) method (Granier, 1985) is widely used to estimate tree transpiration (i.e., the water evaporated from the leaves) because it is simple to build, easy to install, and relatively inexpensive. However, the universality of the original calibration has been questioned and, in many cases, proved to be inaccurate. Thus, when the TDP is used in a new species, specific tests should be carried out. Our aim was to propose a new method for improving the accuracy of TDP on trees in the field. Small hazelnut trees (diameter at breast height 5 cm) were used for the experiment. The response of TDP sensors was compared with a reference water uptake measured with an electronic potometer system provided with a high precision liquid flow meter. We equipped three stems where we measured the sap flow density, the sapwood area (by using fuchsine), the total tree water uptake (reference), and the main meteorological parameters during summer 2018. Results confirmed that the original Granier’s calibration underestimated the effective tree transpiration (relative error about −60%). We proposed a new equation for improving the measurement accuracy within an error of about 4%. The system proposed appeared an easier solution compared to potted trees and particularly suitable for orchards, thus contributing to improve the irrigation management worldwide.

Sap flow in ‘Tommy Atkins’ mango trees under regulated deficit irrigation

Knowledge of transpiration is of fundamental importance for improving irrigation management. This study measured sap flow of the 'Tommy Atkins' mango tree using Granier’s thermal dissipation probe method under regulated deficit irrigation. The work was conducted in a 10-year-old ‘Tommy Atkins’ mango orchard, irrigated by micro sprinkler, located in the Irrigated Perimeter of Ceraíma, in Guanambi, Bahia, Brazil. Sap flow measurements were carried out on three consecutive days in plants under regulated deficit irrigation, with reductions of 30 and 60% of crop evapotranspiration in three phases of fruit development; beginning of flowering to early fruit growth (Phase I), fruit expansion (Phase II) and physiological maturation of fruits (Phase III). Regulated deficit irrigation led to reduced sap flow in ‘Tommy Atkins’ mango tree.

Whole tree water use: Effects of tree morphology and environmental factors

Although tree transpiration (T) studies across multiple spatial scales have been conducted, the synthesis of the driving factors of tree water use, especially for a variety of species under different climatic conditions has not yet been made. This paper analyses T data from 94 published studies conducted in various sites between 1970 and 2016, representing 196 data points to seek relations between morphological traits; tree height (H), diameter at breast height (DBH) and environmental factors; mean annual precipitation (MAP), mean annual temperature (MAT) and altitude (Z) on whole tree water use for 130 species of trees. Techniques used in the studies for T measurement were also analysed. Log transformed T (ln T) varied between 0 and 7.1 L day−1. Univariate correlation and regression analysis revealed that ln T was positively and significantly correlated with H (Spearman correlation coefficient (rs) = 0.55) and DBH (rs = 0.62) at P < 0.1. A weak positive correlation was found between ln T and MAP (rs = 0.16) at P < 0.1. The results further showed that during the study period (1970–2016), 82% of the studies used thermodynamic methods to measure T, in particular thermal heat dissipation probes were used by 60% of the studies, while 21% reported use of heat pulse velocity. The results contribute to a better understanding of T in forest ecosystems, and the factors of control to inform global scale modelling and ecosystem management. Thermodynamic methods, especially thermal heat dissipation probes and heat pulse velocity are the most prevalent techniques used for whole tree T measurement.

Hysteresis in sap flow and its controlling mechanisms for a deciduous broad-leaved tree species in a humid karst region

The critical zone (CZ) represents the intersection of the biosphere with the atmosphere, hydrosphere and lithosphere. Understanding the hydrological processes and human impact factors on the CZ is fundamental to sustainable water resources management for agroforestry. Transpiration ( T ) is an important component of terrestrial evapotranspiration (ET), and understanding the time lag (TL) between vegetation transpiration and meteorological factors can improve our knowledge of the mechanisms of vegetation adaptability to a changing environment. However, the controlling factors on the TL remain poorly understood. Therefore, the objective of this study is identifying the temporal dynamics of key controlling factors on the TL, using a typical deciduous broad-leaved tree species ( Zenia insigins Chun) of CZ in subtropical humid karst regions. This species is used as an example to explore the characteristics of the TL between SF (sap flow) and hydro-meteorological forcing. Sap flow in these 6 trees was monitored using the thermal dissipation probes (TDP). Results showed that: (1) the peak of diurnal sap flow generally lagged behind PAR but preceded Ta (air temperature), RH (relative humidity) and VPD (vapor pressure deficit), with the mean TL of −67.4 min (PAR), 90.5 min ( Ta ), 91.6 min (RH) and 92.9 min (VPD), respectively; (2) TL had no significant relationships with the daily mean meteorological factors and soil moisture, but was highly ( R 2>0.66) correlated to CRs (changing rates of meteorological factors) in the morning; (3) At seasonal scale, the sap flow rate and TL both were controlled by the seasonality of precipitation and temperature. Overall, the seasonality of the TL was caused by plants’ high water loss and strongly active physiological response in hot seasons, leading to close stomata earlier than in cold seasons; (4) The reason why CRs proposed can explain the TL better than mean values of metrological factors is that the CRs considered the distribution and change processes of metrological factors in the daytime. This study may be helpful for understanding the physiological response of vegetation to climatic change, and may be useful for constructing models to simulate transpiration processes more accurately during a day.

Correction of anisotropy effects on penta-needle heat-pulse probe sap-flux density and thermal property measurements

Growing interest in methods for estimating plant stem/trunk sap-flux density and thermal properties include the use of heated needles inserted into the plant. A penta-needle heat-pulse probe (PHPP) coupled with an on-chip integrated INV-WATFLX algorithm was newly developed for inverse estimation of isotropic porous media thermal-diffusivity, κ, -conductivity, λ, and heat velocity, Vh (converted to water-flux density, J), thus heat capacity, C (=λ/κ), and water content could also be derived. This integrated sensor, however, has yet to be applied in anisotropic sapwood sensing. Here, we conducted a numerical simulation of the PHPP heat pulse and a deviation analysis when using an INV-WATFLX code developed by Yang and Jones [Comput. Geosci.—UK. 35 (2009) 2250] in anisotropic porous media. Deviations in J were up to +40% and as low as -30%, and within 12% in κ, λ and C at static conditions for varied PHPP installation angles, α, in sapwood. We developed a correction of anisotropy effects, and followed up with a field test of the sensors installed on standing poplar (Populus simonii Carr.) trees using α = 0°, 15° and 30°. Field tests showed the corrected J estimated using PHPPs at α = 15° and 30° both agreed well with J from thermal dissipation probes (TDPs) in 1:1 line (R2 = 0.87 and 0.83, P < 0.01). The corrected J at α = 0° showed an apparent 30% underestimate (R2 = 0.87, P < 0.01), which was assumed to be due to wound effects. All PHPP estimates exhibited similar and stable κ, λ and C at night, but showed a diurnal fluctuation in J to varying extents likely due to the flow turbulence by inserted needles.

Time lag characteristics of the stem sap flow of Haloxylon ammodendron in the Minqin oasis-desert ectone, China.

The stem sap flow rate of Haloxylon ammodendron plantation in the Minqin oasis-desert ectone was measured by the thermal dissipation probe (TDP). A cross-correlation analysis was used to estimate the time lag between the stem sap flow and the environmental factors influencing transpiration, including photosynthetically active radiation (PAR) and water vapor pressure deficit (VPD). The results showed that the stem sap flow rate of H. ammodendron had substantial seasonal variation, with the monthly average sap flow being the highest in June and the lowest in August. There was an obvious time lag between the stem sap flow of H. ammodendron and PAR and VPD. The stem sap flow was lagged behind PAR for 80 min but it was ahead of VPD for 114 min. Additionally, the time lag exhibited significant difference among different months during the growth season from May to September. The sap flow of H. ammodendron was more dependent on the variation of PAR on the daily scale than VPD, but it was more closely related to VPD during the day time. The time lag between the stem sap flow and PAR/VPD had no significant correlation with the tree factors (including plant height, ground diameter, diameter at 50 cm height, under branch height, canopy size) and the nocturnal sap flow.

Water consumption by an agroecosystem with shelter forests of corn and Populus in the North China Plain

Shelter forests play a significant role in farmland protection and the improvement of microclimates in the North China Plain, but controversy persists over the water relationship between shelter forest and crops. To explore whether the introduction of a protective forest aggravates the water stress of crops, we used stable isotopes to investigating the water sources of corn and its Populus shelter forest. The Penman–Monteith Model and the thermal dissipation probe methods were used to explore the evapotranspiration (ET) of a cornfield with shelter forest (CN) and a cornfield with no shelter forest (CK). The results suggested that corn and Populus exhibited considerable plasticity in depth of water uptake. Although there was no significant difference in water sources of corn between CN and CK, the corn and its shelter forest appeared to have relatively complementary water use patterns. The corn predominantly absorbed water from depths of 0–10 cm (23.4–46.7%), 10–20 cm (33.3–35.8%), and 20–40 cm (20.9–26.3%) during the early growth stages, and it then altered its water sources to depths of 10–20 cm (22.3–24.7%), 20–40 cm (23.7–28.1%), and 40–60 cm (25.7–39.0%) near the of its growth stages. However, Populus mainly obtained water from depths of 40–60 cm (19.8–24.1%), 60–80 cm (24.6–26.9%), and >80 cm (27.3–30.8%) throughout the growing season. The total ET of CN was 6.6% less than that of the CK due to the impact of the microclimate. The ET of CN was primarily from the cornfield, and to a lesser extent, from Populus shelter forest. Our results indicated that the introduction of Populus shelter forest does not increase water stress on corn, and it can reduce regional water resource consumption. For rational utilization of water resources, it would be feasible to select a reasonable arrangement of tree species to develop the Populus shelter forest compound system near corn farmlands in the North China Plain.

Comparing evapotranspiration characteristics and environmental controls for three agroforestry ecosystems in a subtropical humid karst area

Karst landforms account for 10% of the Earths land surface, and it is well-known that water is a limiting factor for karst ecosystems because numerous underground fractures and conduits induce rapid drainage. Therefore, an understanding of ecosystem water consumptions (evapotranspiration) and environmental controls is particularly important for land management on karst landforms. This study compared the actual evapotranspiration (AET) characteristics and analyzed the controls of three agroforestry ecosystems by field observations using a refitted ventilated-chamber, thermal dissipation probes and micro-lysimeters in a subtropical humid karst catchment from 12th March 2015 to 26th May 2016. Results show that: 1) the diurnal transpiration curves of trees plateau at noon with low fluctuations at a high level, while evapotranspiration curves of grass and crop ecosystems present single peaks; 2) AET of the forest-grass mixed ecosystem (7.64 ± 5.75 mm·day−1) was significantly higher than crop (4.24 ± 3.35 mm·day−1) and grass ecosystems (5.78 ± 3.53 mm·day−1); 3) AET of grass and crop ecosystems were most sensitive to temperature, while the forest-grass mixed ecosystem was more sensitive to leaf area index (LAI) than other factors; 4) AET of the forest-grass mixed ecosystem was more sensitive to deep soil moisture (>30 cm), while grass and crop ecosystems were more sensitive to shallow soil moisture (10 cm and 20 cm). This study suggests that soil water content was not key limit factors to AET in humid karst landscapes. However, LAI, which can be controlled by land management, heavily influenced AET rates and E/AET (E, evaporation) ratio through changing water demand.

Water relations in silver birch during springtime: How is sap pressurised?

Positive sap pressures are produced in the xylem of birch trees in boreal conditions during the time between the thawing of the soil and bud break. During this period, xylem embolisms accumulated during wintertime are refilled with water. The mechanism for xylem sap pressurization and its environmental drivers are not well known. We measured xylem sap flow, xylem sap pressure, xylem sap osmotic concentration, xylem and whole stem diameter changes, and stem and root non-structural carbohydrate concentrations, along with meteorological conditions at two sites in Finland during and after the sap pressurisation period. The diurnal dynamics of xylem sap pressure and sap flow during the sap pressurisation period varied, but were more often opposite to the diurnal pattern after bud burst, i.e. sap pressure increased and sap flow rate mostly decreased when temperature increased. Net conversion of soluble sugars to starch in the stem and roots occurred during the sap pressurisation period. Xylem sap osmotic pressure was small in comparison to total sap pressure, and it did not follow changes in environmental conditions or tree water relations. Based on these findings, we suggest that xylem sap pressurisation and embolism refilling occur gradually over a few weeks through water transfer from parenchyma cells to xylem vessels during daytime, and then the parenchyma are refilled mostly during nighttime by water uptake from soil. Possible drivers for water transfer from parenchyma cells to vessels are discussed. Also the functioning of thermal dissipation probes in conditions of changing stem water content is discussed.

The accuracy of the thermal dissipation technique for estimating sap flow is affected by the radial distribution of conduit diameter and density

There are conflicting reports on the accuracy of the thermal dissipation probe (TDP, the Granier method) measurement using the original formula, which is widely used to estimate the transpiration of individual trees and forest stands. In this article, six woody species of three wood types were used to study a possible association between TDP measurement accuracy and wood anatomical characteristics, including the vessel diameter and density, as well as sapwood depth. We found that TDP technique with Granier’s original equation underestimated the sap flux density in six species to various degrees, dependent on conduit size and sap flux. Our calibration using two conifers with small diameters and a high density of tracheids was relatively consistent with Granier’s calibration; however, because there were larger diameters and lower densities of vessels in the two diffuse-porous species, the original calibration significantly underestimated sap flow. Two ring-porous species had the largest diameters and lowest densities of vessels. In particular, Robinia pseudoacacia possessed the shallowest sap wood depth, less than a probe length. Our calibration for the ring-porous species, especially R. pseudoacacia, deviated far from the original calibration, which mostly underestimated the sap flow. The degree of underestimation was well associated with sap wood depth and the radial diameter and density distribution of conduits. Our results demonstrated that a new calibration must be operated for each species together with the sapwood depth determination and more probes may be applied for one stem in the field to obtain the more accurate sap flux. In addition, we investigated the effects of different environmental temperature and perfusing fluid composition on the TDP-based sap flux measurement. We found that an environmental temperature reduction from 25 to 0 °C did not alter the values of the maximum temperature difference (ΔTm) between a heated probe and a reference probe when there was no sap flow, verifying that ΔTm measured at night can be used as a reference in daytime.

Responses of sap flow to natural rainfall and continuous drought of tree species growing on bedrock outcrops

This study focused on bedrock outcrops, a very common habitat in karst region of southwest China. To reveal the responses of plant transpiration to natural rainfall and continuous drought, two tree species typical to this habitat, Radermachera sinica and Triadica rotundifolia, were selected as test materials. A rainout shelter was used to simulate continuous drought. The sap flow dynamics were monitored using the method of Granier's thermal dissipation probe (TDP). Our results showed that sap flow density increased to different degrees after rain in different stages of the growing season. Sap flow density of the deciduous species T. rotundifolia was always higher than that of the semi-deciduous species R. sinica. After two months without rainfall input, both species exhibited no obvious decrease in sap flow density, indicating that rainfall was not the dominant source for their water uptake, at least in the short-term. Based on the regression relationships between sap flow density and meteorological factors before and after rainfall, as well as at different stages of continuous drought, we found that the dynamics of meteorological factors contributed little to plant transpiration. The basic transpiration characteristics of both species were not changed in the circumstance of natural rainfall and short-term continuous drought, which would be closely related to the special water storage environments of bedrock outcrops and the reliance on deep water sources by tree species.

Stand Transpiration Estimates from Recalibrated Parameters for the Granier Equation in a Chinese Fir (Cunninghamia lanceolata) Plantation in Southern China

Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) is an important native tree species that is widely distributed in subtropical areas of southern China and cultivated for wood extraction. However, information on water use by Chinese fir plantations is still scarce. In this study, we performed species-specific parameter calibrations for the original Granier equation for sap flow density (SFD) estimates. Stand transpiration (Es) was related to environmental inputs, such as rainfall, air temperature (Ta), vapor pressure deficit (VPD), photosynthetically-active radiation (PAR), air relative humidity (RH), and potential evapotranspiration (PET) in order to examine how environmental factors affect the Es of Chinese fir plantations. According to our results, Granier’s original parameters set underestimated C. lanceolata SFD by up to 54% compared to our species-specific calibrated parameters set. A strong positive correlation was found between SFD, diameter at breast height (DBH), and tree height among trees populations. The total Es in 2015 was 522.1 mm, with obvious seasonal dynamics and an average of 1.4 mm·day−1. Daily and monthly Es were positively correlated with VPD, PAR, Ta, and PET. A negative relationship between Es and RH was detected only at a daily timescale. Our findings indicate that the original Granier equation requires a parameter calibration when it is applied to species-specific thermal dissipation probe (TDP) estimates, and our findings can also provide novel insights on the water use of Chinese fir in major wood production areas in Southern China.

Sap flow characteristics of Quercus liaotungensis in response to sapwood area and soil moisture in the loess hilly region, China

To examine the characteristics of sap flow in Quercus liaotungensis and their response to environmental factors under different soil moisture conditions, Granier-type thermal dissipation probes were used to measure xylem sap flow of trees with different sapwood area in a natural Q. liaotungensis forest in the loess hilly region. Solar radiation, air temperature, relative air humidity, precipitation, and soil moisture were monitored during the study period. The results showed that sap flux of Q. liaotungensis reached daily peaks earlier than solar radiation and vapor pressure deficit. The diurnal dynamics of sap flux showed a similar pattern to those of the environmental factors. Trees had larger sap flux during the period with higher soil moisture. Under the same soil moisture conditions, trees with larger diameter and sapwood areas had significantly higher sap flux than those with smaller diameter and sapwood areas. Sap flux could be fitted with vapor pressure deficit, solar radiation, and the integrated index of the two factors using exponential saturation function. Differences in the fitted curves and parameters suggested that sap flux tended to reach saturation faster under higher soil moisture. Furthermore, trees in the smaller diameter class were more sensitive to the changes of soil moisture. The ratio of daily sap flux per unit vapor pressure deficit under lower soil moisture condition to that under higher soil moisture condition was linearly correlated to sapwood area. The regressive slope in smaller diameter class was larger than that in bigger diameter class, which further indicated the higher sensitivity of trees with smaller diameter class to soil moisture. These results indicated that wider sapwood of larger diameter class provided a buffer against drought stress.

Sap flow of black locust in response to environmental factors in two soils developed from different parent materials in the lithoid mountainous area of North China

Relative extractable water and reference evapotranspiration rate were the main environmental factors affecting sap flow in black locust trees in China, while soil parent material also influenced trees’ water use. Whole-plant water use is affected by meteorological factors, soil water availability, and soil texture. Although water use by plants growing in lithoid mountainous areas with thin soil layers has been studied, the effect of soil parent material on whole-plant water use remained unclear. In the present study, we used thermal dissipation probe sensors to examine the effects of soil water availability and various meteorological factors on black locust sap flow in cinnamon and brown soils, originating from different parent materials. The results showed that vapour pressure deficit (VPD) and reference evapotranspiration rate (ET0) were more closely correlated with sap flow than solar radiation (Rs) and relative humidity (RH), but that Rs and ET0 explained more of the variation in sap flow than did the other two factors. Decreasing ET0 and VPD was produced a more sensitive sap flow response in trees that were growing in the cinnamon soil than in those growing in the brown soil, whereas the sap flow response to relative extractable water (REW) was less sensitive in the cinnamon soil than in the brown soil. REW and ET0 were the main environmental factors affecting sap flow. Furthermore, soil water content in cinnamon soil was not the only water source available to trees, as the exponential curve explained only 56% of the relationship between sap flow and REW. This finding indicated that soil parent material may have an important influence on plant water utilization. Future studies of plant water use should devote more attention to soil parent materials.

Responses of Sap Flow of Deciduous and Conifer Trees to Soil Drying in a Subalpine Forest

Co-occurring species may adopt different water-use strategies to adapt to limited soil water. In Jiuzhaigou Valley, a continuous decline in soil water after an initial recharge from the thawing of snow and frozen soil in early spring was observed, but its effects on the sap flow dynamics of co-occurring species are not well understood. To clarify the species-specific water-use strategy, variations in sap flow and environmental conditions were investigated for two co-occurring species (Betula albosinensis Burk. and Pinus tabuliaeformis Carr.) in a mixed forest during a transition from the wet to dry period in 2014. Sap flow was measured using Granier-type thermal dissipation probes, and the soil-water content was measured using time-domain reflectometry probes for a successive period. Our study showed that B. albosinensis maintained relatively high transpiration until late into the season regardless of soil moisture, while the transpiration of P. tabuliformis showed a continuous decrease in response to seasonal soil drying. Sap flow for both species exhibited a marked hysteresis in response to meteorological factors and it was conditioned by the soil-water status, especially in the afternoon. We found that P. tabuliformis was sensitive to soil-water conditions, while for B. albosinensis, the sap flow was not very sensitive to changes in soil-water conditions. These results indicate that B. albosinensis could manage the water consumption conservatively under both dry and wet conditions. These results may have implications for evaluating the species-specific water-use strategy and carrying out proper reforestation practices.

Potential Sources of Errors in Estimating Plant Sap Flow Using Commercial Thermal Dissipation Probes

Thermal dissipation systems are a conventional technique to estimate sap velocity by determining the temperature difference between sap flow probes. The Thermal Dissipation Probe (TDP) method is commercially available and has been used by many researchers and professionals to estimate sap flow in plants. However, some errors and practical issues cause inaccuracy when evaluating plant sap velocity with this technique. Specifically, the sources of these errors are from the effect of the ambient thermal gradient (i.e., error caused by the cold junction location), the underestimation of nighttime sap flow, and the deficient thermal contact between the probe and tree body. This article focuses on errors associated with the traditional TDP, which effect on the thermal difference between the reference and heated probes. These errors vary as a nonlinear function of the transmitter length and the numbers of cold junctions which are affected by changes in the ambient temperature. Keywords: Ambient thermal gradient, Sap velocity, Thermal dissipation probe (TDP), Thermocouple cold junction.